Electrolytic process for the recovery of nickel, cobalt and iron from their sulfides

ABSTRACT

A pollution-free process for the electrolytic dissolution of sulfide concentrates of the metals of Group VIII of the Periodic Table in aqueous acidic media with the formation of metal ions and elemental sulfur followed by recovery of the metal ion from solution in the electrolyte media, the process characterized by certain critical process conditions, these being the use of: 1. AN ALKALI METAL AND/OR ALKALINE EARTH METAL CHLORIDE ELECTROLYTE BEING ABOVE ABOUT 0.5N to saturation in chloride ion, 2. A SULFIDE FEED OF AVERAGE PARTICLE SIZE SMALLER THAN 60 MESH U.S. Standard, 3. A PH range of about 0.01 - 3.9, 4. AN ELECTROLYTE TEMPERATURE OF ABOUT 50*C - 105* C, and 5. AN ANODE CURRENT DENSITY ABOVE ABOUT 12 AMPERES/FT2.

nited States Patent [1 1 Kruesi [75] Inventor: Paul R. Kruesi, Golden,C010.

[73] Assignee: Cyprus Metallurgical Processes Corporation, Los Angeles,Calif.

[22] Filed: May 10, 1972 [21] Appl. No.: 251,940

[52] U.S. Cl. 204/113, 204/128 [51] Int. Cl...... C22d l/14, C22d 1/24,COlb 17/06 [58] Field of Search 204/113, 128

[56] References Cited UNITED STATES PATENTS 967,996 8/1910 Summers204/130 840,511 l/l907 Packard 204/130 3,673,061 6/1972 Kruesi 204/105 R3,464,904 9/1969 Brace 204/105 R FOREIGN PATENTS 0R APPLICATIONS 556,1694/1958 Canada 204/128 Oct. 16, 1973 Primary Examiner-John H. MackAssistant Examiner-R. L. Andrews Attorney-Sheridan, Ross & Fields [5 7]ABSTRACT A pollution-free process for the electrolytic dissolution ofsulfide concentrates of the metals of Group VIII of the Periodic Tablein aqueous acidic media with the formation of metal ions and elementalsulfur followed by recovery of the metal ion from solution in theelectrolyte media, the process characterized by certain critical processconditions, these being the use of:

1. an alkali metal and/or alkaline earth metal chloride electrolytebeing above about 0.5N to saturation in chloride ion,

2. a sulfide feed of average particle size smaller than 60 mesh U.S.Standard,

3. a pH range of about 0.01 3.9,

4. an electrolyte temperature of about 50C 105 C, and

5. an anode current density above about 12 amperes/ft 15 Claims, NoDrawings ELECTROLYTIC PROCESS FOR THE RECOVERY OF NICKEL, COBALT ANDIRON FROM THEIR SULFIDES BACKGROUND OF THE INVENTION Metals of GroupVIII of the Periodic Table have been conventionally recovered from theirsulfide concentrates by pyrometallurgical smelting techniques. This hasrequired a high degree of concentration for economic processingparticularly for nickel and cobalt and thus low grade concentrates orcomplex concentrates which were not amenable to physical separation areoften considered valueless or of low value.

The pyrometallurgical processes in which sulfur contained in the ores isoxidized to sulfur dioxide of which a substantial proportion is releasedto the atmosphere with consequent damage to the environment are a causeof substantial concern. An electrolytic process requiring only economicquantities of power, in which substantially all of the sulfur in theabove metal sulfides is converted to elemental sulfur is an answer tothis air pollution problem.

US. Pat. No. 2,839,461 describes an electrolytic process for therecovery of nickel from nickel sulfide utilizing an acidsulfate-chloride electrolyte and which is dependent upon the anodecurrent being passed through a nickel matte anode. The process has thedisadvantage of requiring concentrates suitable for the production ofthe required anode matte and is subject to the substantial expense ofanode preparation and removal from the cells with subsequent treatment.

The process herein disclosed has the advantage over priorpyrometallurgical process of converting the sulfide sulfur to elementalsulfur rather than sulfur dioxide with its attendant air pollution andthe advantage over the process disclosed in US. Pat. No. 2,839,461 ofbeing adaptable to low grade and complex concentrates, and not requiringthe production of a matte anode in the case of nickel and cobalt.

STATEMENT OF THE INVENTION The term metal sulfide as contained herein isinclusive of the complex as well as the simple sulfide minerals whichcontain economically recoverable amounts of the specified metals.

The invention is a pollution-free process for recovery of metals ofGroup VIII of the Periodic Table from their sulfide and mixed sulfideores in which the sulfide is electrochemically dissociated in an acidaqueous media into elemental sulfur and metal ions which are thenrecovered from solution in the electrolyte media by conventionaltechniques. The electrolysis process is characterized by certaincritical process conditions which render it economically feasible, thesebeing the use of:

1. an alkali or alkaline earth metal chloride electrolyte,

2. a sulfide feed particle size smaller than about 60 mesh U.S.Standard,

3. a pH range below about 3.9,

4. an electrolyte temperature range of about 50 105 C, and

5. an anode current density above about 12 ampereslft DETAILEDDESCRIPTION OF THE INVENTION The economic feasibility of the process isdependent upon the current required to produce a given quantity ofmetal. It is expressed herein in terms of the ampere hours of currentrequired to release a pound of metal. Because nickel ores frequentlycontain substantial quantities of iron and copper which may becoproducts, the efficiency here is expressed as the ampere hoursnecessary to release one pound of the combined metal.

The process parameters which have been found to control the currentrequirement for the process are electrolyte composition, feed particlesize, operating acidity, operating temperature, and anode currentdensity. As the examples which follow show, these factors are mutuallyinteracting and dependent as respects their effect on currentrequirements.

The electrolytic media for the process must be acidic as an alkalineelectrolyte has proven unsatisfactory. Elemental sulfur is not stable inan alkaline environment because oxidation of the sulfur proceeds rapidlyin this media through thiosulfate, hydrosulfite, sulfite to sulfate. Thepresence of sulfate ions is undesirable because at high sulfateconcentrations oxygen is rapidly evolved at the anode resulting in adecrease in current efficiency.

The preferred electrolyte media is an aqueous acidic solution of alkalimetal chloride or alkaline earth metal chloride or mixtures thereof. Thechlorides of sodium, potassium, barium and calcium or mixtures thereof,have been found suitable. Concentrations from 0.5N to saturation may beused. Voltage across the cell is lower at higher salt concentrations sothat these are preferred except where low grade feeds are used and wheresalt losses would therefore become significant.

It is highly important that a high percentage of the sulfur in the metalsulfide be recovered as elemental sulfur both from the standpoint ofpollution control and from the electrical efficiency of the process.Each mole of sulfur which is oxidized from elemental sulfur to sulfaterather than being converted to elemental sulfur requires six Faradayswhich is equivalent to 2,275 ampere hours per pound of sulfur. Thisrenders oxidation of the sulfur to sulfate prohibitive for aneconomically acceptable process.

The particle size of the feed material is critical as it directlyeffects the conversion to elemental sulfur. The sulfur produced isextremely fine. The anode current attacks the metal sulfidepreferentially to sulfur provided the sulfide has sufficient activitynear the anode. The activity of the sulfide is a function of itsconcentration and its exposed surface area. Therefore the presence of ahigh concentration of fine sulfide near the anode prevents thecontinuing oxidation of sulfur and results in higher efficiency andconsequently lower current consumption. An average grain size range forthe feed sulfide smaller than about 60 mesh US. Standard is the operablerange.

An acidity for the electrolyte media up to about 3.9 is critical.Current efficiency is reduced at a pH above 3.9. The preferred acidityis about pH 0.5.

The reaction temperature of the electrolyte is critical and high processefficiency is not obtained at low temperature. At temperature belowabout 50 C, the reaction for the conversion of sulfide to sulfate ratherthan to sulfur is increasingly favored. A temperature range of about 50to 105 C is the operable range. A temperature of about C is mostpreferred.

Anode current density is also critical as used with the other criticalparameters with a preferred range being between 12 amperes/ft and 240amperes/ft with the minimum being about 12 amperes/ft? While highefficiency may be maintained at relatively high current densities whenample fresh feed is present, it is necessary to decrease current densityas the concentration of sulfur becomes high in proportion to that of themineral being attacked.

The metal dissolved in the electrolyte can be finally recovered byconventional methods such as, electrolysis, precipitation, cementation,etc. Under certain conditions the metal can be plated out on the cathodeduring the dissociation process and recovered in this manner.

Elemental sulfur is readily recovered from the electrolyte media by theprocess disclosed in co-pending patent application Ser. No. 233,352filed in the US Patent Office on Mar. 9, 1972, William G. Kazel,entitled Sulfur Recovery Process.

The following examples are illustrative of the invention but notlimiting thereof.

The examples were performed in apparatus well known in the artconsisting of an anode section provided with means for agitation, asuitable anode, a cloth diaphragm and a suitable cathode in a cathodesection. Conventional non-diaphragm cells may be used.

The feed concentrate for all of the examples was 60 mesh US. Standard.Current density is given in amperes per square foot. Current requirementis expressed in amperes per pound of metal or combined metal recovered.

1n Examples 1-4, 400 grams of a nickel sulfide concentrate assaying 8.3percent nickel, 5.2 percent copper, 37.8 percent iron were slurried in 2liters of electrolyte and subjected to 60 ampere hours of current.

EXAMPLE 1 The following tests were performed to determine theapproximate lower limit of the operable temperature range for recoveringnickel and iron from nickel sulfide using a sodium chloride electrolyteand other parameters of the process.

TEST NO. 1 2 3 4N 4N 4N Electrolyte NaCl NaCl NaCl Temperature 80C' 50C30C pH 0.5 0.5 0.5 Anode Current Density (Amps/ft) 120 120 120 Metal andSulfur Recovered (gm.)

Fe 69.5 27.7- 21.0 Ni 6.5 3.1 2.8 Cu 6.4 1.3 3.3 S 42.2 1 1.4 7.8 Amp.Hrs./lb. Combined Metals 330.5 848.5 1005.2 Recovered The tests showthat reduction of the operating temperature below about 50 C results inexcessive current requirements and drastic reduction in the conversionof sulfide sulfur to elemental sulfur.

EXAMPLE 2 The following tests were selected to show the effect of pH onthe recovery of nickel and iron from nickel sulfide by the process.

TEST NO. 1 2 3 4 5 Electrolyte NaCl NaCl NaCl NaCl NuCl Temperature C80C 80C 80C 80C pH 0.01(5%HC|) 0.5 1.5 2.0 4.0 Anode Current DensityAmps/ft 120 120 120 120 Metal and Sulfur Recovered (gm) Fe 71.8 69.551.3 37.8 9.8 Ni 5.6 6.5 3.2 1.2 0.6 Cu 3.5 6.4 5.6 1.1 0.5 S 52.8 42.236.3 28.6 22.2 Amp. Hrs/1b. Combined Metals 336.9 330.5 453.1 679.22499.3 Recovered The increase in current requirement and decrease inconversion of sulfide sulfur to elemental sulfur above pH 1.5 as aciditydecreases indicates the preferred pH. Above pH 3.9, the upper limit ofpH range, 2,499.3 amperes/lb. of metal recovered were required andsulfur conversion was reduced to 22.2 grams.

EXAMPLE 3 The following tests were made to explore the preferred currentdensity range for the process.

TEST NO. 1 2 3 4 4N 4N 4N 4N Electrolyte NaCl NaCl NaCl NaCl Temperature80C 80C 80C 80C p 0.5 0.5 0.5 0.5 Anode Current Density(Amps/ft 480 240120 60 Metal and Sulfur Recovered (gm.)

Fe 38.8 36.2 69.5 66.1 Ni 4.0 4.4 6.5 5.0 Cu 5.2 6.3 6.4 3.7 S 24.2 27.042.2 39.0 Amp. Hrs/lb. Combined Metals Recovered 567.5 580.7 330.5 364.1

The results indicate that current densities at or below about 120amps/ft are preferable to those above this figure. The lower limit ofthe economically feasible current density range is about 12 amps/ft?EXAMPLE 4 The tests below were performed to explore the effectiveness ofother electrolytes than NaCl and to determine the effect of the presenceof sulfate ion in the electrolyte on the operativeness of the processfor recovering nickel and iron from nickel sulfide concentrate.

The tests show that potassium chloride as well as the alkaline earthmetal chlorides, calcium chloride and magnesium chloride, are effectiveelectrolytes for the process. In Test No. 5 the sodium sulfate-sodiumchloride electrolyte of U.S. Pat. No. 2,839,461 is shown to be lesseffective than the other electrolytes as indicated by the decrease insulfur conversion.

EXAMPLE 5 The following test was selected to show the operativeness ofthe process for the recovery of cobalt, nickel and iron from theirsulfides. 400 grams of nickel sulfide concentrate assaying 8.3 percentnickel, 5.2 percent copper, 37.8 percent iron and 0.337 percent cobaltwere slurried in two liters of electrolyte and subjected to 90 amperehours of current.

Electrolyte 4N NaCl Temperature 80C p 0.5 Anode Current Density(Amps/ft)60 Metal and Sulfur of Feed Metal Recovered (gms.) Recovered Fe 88.761.8 Ni 16.6 54.1 Cu 9.35 49.1 Co 0.63 51.7 S 58.4 Amp.Hrs./lb.

Combined Metals Recovered 354.6

As the results indicate, the process is effective for the recovery ofcobalt as well as iron and nickel from their sulfides with goodconversion of sulfur and satisfactory current requirements.

EXAMPLE 6 Electrolyte 4N NaCl Temperature 80C p 0.01 5% HCl) AnodeCurrent Density (Amps/ft) l20 Metal and Sulfur Recovered (gms) Co 0.5 Ni0.2

S 19.9 Amps. HrsJlb. Combined Metals Recovered 472.2

The high conversion of sulfur and the relatively low currentrequirements with satisfactory recovery of cobalt show that the processcan be used economically for the recovery of cobalt from low gradecomplex cobalt sulfide ores.

The power requirements set forth in the examples are well withincommercial feasibility ranges for large scale production of nickel, ironand cobalt from their sulfide and mixed sulfide ores. The cost of therecovery of the metals and sulfur from the electrolyte afterelectrolysis by conventional techniques is comparatively small. The highpercentage recovery of sulfur from the sulfides as elemental sulfursubstantially reduces or eliminates the pollution problems associatedwith prior art processes. Accordingly, the invention provides a processfor the recovery of nickel and cobalt from their sulfide and mixedsulfide ores which has the advantages of being commercially feasible andpollution free.

I claim:

1. A process for the recovery of metals selected from the groupconsisting of iron, nickel and cobalt from their sulfides and mixedsulfides, and mixtures thereof, by electrolytic dissolution with theformation of elemental sulfur, which process comprises:

a. providing an electrolyte in an electrolytic cell including at leastan anode and a cathode, the electrolyte comprising an acidic aqueoussolution of at least one chloride salt selected from the groupconsisting of alkali metal chlorides and alkaline earth metal chlorides,the solution having a concentration from about 0.5N to saturation;

b. mixing with the electrolyte a solid feed sulfide of the metal havingan average particle size smaller than about 60 mesh U.S. Standard;

c. maintaining the temperature of the electrolyte media at about 50 toC, and the pH of the electrolyte media below about 3.9 while introducingelectric current into the electrolytic cell to provide an anode currentdensity above about 12 amperes per square foot to dissociate the metalsulfide into metal ions and elemental sulfur; and

d. recovering the metal from the electrolyte.

2. The process of claim l in which cobalt and nickel are recovered fromthe sulfides in the presence of iron sulfides.

3. The process of claim 1 including the final step of recovering themetal from solution in the electrolyte by electrode-position on thecathode.

4..The process of claim it including the step of recovering elementalsulfur from the electrolyte.

5. The process of claim 1 in which the metal recovered is nickel.

6. The process of claim 1 in which the metal recovered is cobalt.

7. The process of claim 1 in which the metal recovered is iron.

8. The process of claim l in which the alkali metal chlorides are sodiumand potassium chlorides and the alkaline earth metal chlorides arecalcium and magnesium chlorides.

9. The process of claim 8 in which the electrolyte is sodium chlorideand the metal'recovered is a metal selected from the group consisting ofiron, nickel and cobalt.

10. The process of claim 9 in which the metal recovered is nickel.

11. The process of claim 9 in which the metal recovered is cobalt.

12. The process of claim 9 in which the metal recovered is iron.

13. The process-of claim 8 in which the alkali metal chloride ispotassium and the metals recovered are nickel and iron.

14. The process of claim 8 in which the alkaline earth metal chloride iscalcium chloride and the metals recovered are nickel and iron.

15. The process of claim 8 in which the alkaline earth metal chloride ismagnesium chloride and the metals recovered are nickel and iron.

1. AND ALKALI METAL AND/OR ALKALINE EARTH METAL CHLORIDE ELECTROLYTEBEING ABOVE ABOUT 0.5N TO SATURATION IN CHLORIDE ION,
 2. A SULFIDE FEEDOF AVERAGE PARTICLE SIZE SMALLER THAN 60 MESH U.S. STANDARD,
 2. Theprocess of claim 1 in which cobalt and nickel are recovered from thesulfides in the presence of iron sulfides.
 3. The process of claim 1including the final step of recovering the metal from solution in theelectrolyte by electrode-position on the cathode.
 3. A PH RANGE OF ABOUT0.01-3.9,
 4. AND ELECTROLYTE TEMPERATURE OF ABOUT 50*C-105*C, AND
 4. Theprocess of claim 1 including the step of recovering elemental sulfurfrom the electrolyte.
 5. The process of claim 1 in which the metalrecovered is nickel.
 5. AN ANODE CURRENT DENSITY ABOVE ABOUT 12AMPERES-FT2.
 6. The process of claim 1 in which the metal recovered iscobalt.
 7. The process of claim 1 in which the metal recovered is iron.8. The process of claim 1 in which the alkali metal chlorides are sodiumand potassium chlorides and the alkaline earth metal chlorides arecalcium and magnesium chlorides.
 9. The process of claim 8 in which theelectrolyte is sodium chloride and the metal recovered is a metalselected from the group consisting of iron, nickel and cobalt.
 10. Theprocess of claim 9 in which the metal recovered is nickel.
 11. Theprocess of claim 9 in which the metal recovered is cobalt.
 12. Theprocess of claim 9 in which the metal recovered is iron.
 13. The processof claim 8 in which the alkali metal chloride is potassium and themetals recovered are nickel and iron.
 14. The process of claim 8 inwhich the alkaline earth metal chloride is calcium chloride and themetals recovered are nickel and iron.
 15. The process of claim 8 inwhich the alkaline earth metal chloride is magnesium chloride and themetals recovered are nickel and iron.